linux-kernel - Re: [PATCH v5 6/6] mm: handle userfaults under VMA lock

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Message-ID: <ZJ2yOACwp7B2poIw@x1n>
Date:   Thu, 29 Jun 2023 12:32:56 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, hannes@...xchg.org,
        mhocko@...e.com, josef@...icpanda.com, jack@...e.cz,
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        viro@...iv.linux.org.uk, brauner@...nel.org,
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        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v5 6/6] mm: handle userfaults under VMA lock

On Wed, Jun 28, 2023 at 05:19:31PM -0700, Suren Baghdasaryan wrote:
> On Wed, Jun 28, 2023 at 10:32 AM Peter Xu <peterx@...hat.com> wrote:
> >
> > On Wed, Jun 28, 2023 at 10:25:29AM -0700, Suren Baghdasaryan wrote:
> > > Enable handle_userfault to operate under VMA lock by releasing VMA lock
> > > instead of mmap_lock and retrying. Note that FAULT_FLAG_RETRY_NOWAIT
> > > should never be used when handling faults under per-VMA lock protection
> > > because that would break the assumption that lock is dropped on retry.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> >
> > Maybe the sanitize_fault_flags() changes suite more in patch 3, but not a
> > big deal I guess.
> 
> IIUC FAULT_FLAG_RETRY_NOWAIT comes into play in this patchset only in
> the context of uffds, therefore that check seems to be needed when we
> enable per-VMA lock uffd support, which is this patch. Does that make
> sense?

I don't see why uffd is special in this regard, as e.g. swap also checks
NOWAIT when folio_lock_or_retry() so I assume it's also used there.

IMHO the "NOWAIT should never apply with VMA_LOCK so far" assumption starts
from patch 3 where it conditionally releases the vma lock when
!(RETRY|COMPLETE); that is the real place where it can start to go wrong if
anyone breaks the assumption.

Thanks,

-- 
Peter Xu